(1) Field of the Invention
The present invention relates to an image processing apparatus which is used in digital copiers, scanners, etc., and which effects optimal density correction to the image data scanned from an original, in conformity with the features of the original.
(2) Description of the Prior Art
Conventionally, digital copiers and scanners scan the image of an original using a CCD (charge coupled device), image sensor etc., and effect various image processings to the image data obtained by the scanning operation in order to improve the quality of the recorded image which will be reproduced from the image data.
The image processing for the purpose of improving the recorded image includes an operation for optimal correction of density in conformity with the features of the scanned original. For example, when an original of newspaper etc. is scanned, a density correction for deleting the gray tone of the background and the image of text on the reverse side will be performed in order to improve the visual appearance of the reproduced image, thus providing improved images for viewers. It is also well known that when an original drawn in pencil etc. is scanned, its density is corrected so as to enhance thin text so that the reader can easily recognize the reproduced image.
One example of such methods of density correction has been proposed in Japanese Patent Application Laid-Open Hei 5 No.236,277 `an image processing apparatus`. This disclosure proposes a method of density correction to the image data which comprises the steps of preparing histograms from the image data (image signal) picked up from the original, analyzing the features (the lightest level, the darkest level, the modal point of the distribution, the level at the modal point, etc.) to identify the features of the original, transforming the output levels of the image data depending upon the transformation table prepared in accordance with the feature information.
However, in accordance with the image processing apparatus of performing density correction disclosed in Japanese Patent Application Laid-Open Hei 5 No.236,277, if it is applied to a digital copier, the original needs to be pre-scanned before the actual scanning of the image data, resulting in slow operation of the image processing. Otherwise, if the original is attempted to be identified exactly without performing the pre-scanning, a large capacity of storage for storing the image data of the original is needed.
Density transforming tables are prepared in association with the types of originals, by recognizing the type of an original based on threshold processing analysis of the feature data of the original. However, the threshold processing analysis is not enough to precisely identify the type of original, so that it is difficult to produce a density transforming table which reflects the detail of the characteristics of the feature data of the original.
As a result, it is impossible to exactly classify all types of originals and perform density correction based on the features of the original, thus possibly causing degradation of the recorded image for some types of originals.